Production of chromium/chromium alloys using a reverse-polarity thermal plasma-fired reactor

A fundamental investigation of a novel process for production of chromium metal from low-grade chromium-bearing ore using a reverse-polarity transferred-arc thermal plasma-fired reactor has been studied. The major objective of this work was to investigate the feasibility of the reduction of chromic oxide by a reverse-polarity DC plasma-driven molten salt electrolysis process. Electrolysis was performed in a laboratory-scale cold crucible system. Such parameters as starting slag composition, plasma torch voltage drop, and cathode current density were investigated in the experiments. Several different starting slag systems were tested. They were Cr2O3-BaO, Cr2O 3-CaO, Cr2O3-CaO-CaF2, SiO2-CaO-Al 2O3-Cr2O3-Na2O and SiO 2-CaO-Cr2O3-Na2O. Chromium metal ingots were successfully produced. Among the slag systems tested, SiO2-CaO-Al 2O3-Cr2O3-Na2O and SiO 2-CaOCr2O3-Na2O both seemed to be promising. Therefore, these two systems were a specific focus. Experimental tests demonstrate that this process may be both economically and technically advantageous over the currently existing technologies. To study oxygen evolution rate from the anode, carbon monoxide gas was introduced to the electrolysis system. For SiO2-CaO-Al2O3-Cr2O3-Na 2O system, oxygen evolution rate showed a maxima curve during the electrolysis process. For SiO2-CaO-Cr2O3-Na2O system, oxygen evolution rate displayed a declining curve with the processing time. Based on these observations, two different kinetic mechanisms, which are oxygen ion diffusion controlling model and electrochemical reaction controlling model, were proposed. The metal products were analyzed by the ICP method. It was found that aluminum was also reduced with the presence of alumina in the starting slag. This fact demonstrates that the new process can be used to produce various desired final metal compositions such as chromium metal and chromium alloys depending on the starting metal oxides fed into the molten bath. The significance of this process is that it can be used to produce carbon-free chromium/chromium alloys. A descriptive model that simulates the reverse-polarity thermal plasma-driven molten salt electrolysis was formulated and solved. Such parameters as temperature profile of the molten bath, chromic oxide concentration in the molten electrolyte, oxygen evolution rate, and slag-metal boundary moving history were obtained from the model. Also theoretical calculation results were compared with the experimental results. Comparison between numerical simulation and experimental data showed good agreement.